Why sizeof(MyStruct) is often larger than the sum of its member variables:

struct UnalignedData {
    char a;     // 1 byte
    // 7 bytes of padding inserted by compiler!
    double b;   // 8 bytes (must align to 8-byte boundary)
    int c;      // 4 bytes
    // 4 bytes of tail padding!
}; // Total: 24 bytes

By reordering members from largest to smallest (double b; int c; char a;), the total struct size drops to 16 bytes without changing functionality, saving 33% memory and improving CPU cache line efficiency.

Reordering struct fields by descending alignment is one of the easiest zero-cost memory optimizations you can do in C/C++.

Also useful when interfacing with AVX SIMD: using alignas(32) ensures 256-bit vectors can use aligned load instructions _mm256_load_ps instead of slower unaligned loads.